1. Field of the Invention
The present invention relates to compositions and low temperature sintering methods of making ceramic bodies that have low thermal expansion and low porosity.
2. Description of the Prior Art
Porcelain is traditionally made from mixtures of clay, quartz and flux. The clay imparts plasticity and dry strength, the quartz acts as filler, and the flux allows the mixture to fuse more readily. The mixture is vitrified at high temperatures to make high impact strength, low porosity, low water absorption bodies that have been used in a variety of applications including reaction vessels, spark plugs, electrical resistors, corrosion-resistant equipment, pumps, valves, food processing equipment and tableware.
Although porcelain and other ceramics offer many desirable physical properties, the materials are traditionally subject to thermal shock. Thermal shock occurs when a material undergoes a large and rapid change in temperature and the corresponding change in density is too great to be accommodated quickly enough. The result is that the material cracks or breaks. Resistance to thermal shock is reflected in a material's low thermal expansion or low thermal expansion coefficient. Thus, for many applications involving rapid temperature changes, materials having a very low thermal expansion coefficient have been sought.
Applications requiring a ceramic with a very low thermal expansion coefficient include heat exchangers, stovetops, bake ware, catalytic converters and burner nozzles. Ceramic bake ware having a high thermal shock resistance is very desirable because it can be transferred from freezer to oven or from oven to countertop without cracking the ceramic. In addition to thermal shock resistance, such bake ware should have low porosity and low water absorption in order to be stain resistant and dishwasher safe. Current bake stones and similar bake ware do not have sufficiently low porosity to be used in dishwashers. The porosity of current, available bake stones is such that water and soap used in washing will penetrate and/or be absorbed into the bake stone.
Li2O—Al2O3—SiO2 (LAS) based ceramics with very low thermal expansion have been developed. For example, beta spodumene (Li2O—Al2O3—4SiO2) bodies have been produced having a thermal expansion coefficient of less than 1.0×10−6/° C. Methods for producing beta spodumene ceramics include crystallizing beta spodumene from lithium bearing aluminosilicate glass shapes, sintering shapes formed from beta spodumene produced by sol gel methods, and sintering mixtures of ground beta spodumene glass and minerals. Each of these methods of making LAS based ceramic materials involve additional steps that are often comparatively energy intensive. They also require conventional firing at high temperatures, which is itself energy intensive and increases refractory wear in the kilns compared to low temperature firing.
Thus there are continuing needs for low temperature fired ceramics having low porosity and low thermal expansion coefficients.